Many environments have high levels of electromagnetic radiation due to a concentration of transmitting devices. Additionally, other types of electrical equipment generate leakage of electromagnetic radiation when they operate even though it is not their function to transmit radiation. Unwanted radiation from sources such as these may interfere with the operation of many pieces of electrical equipment. It is thus important to test electrical equipment to determine how well it operates when it is subjected to unwanted electromagnetic radiation.
Electromagnetic field test chambers were developed to provide shielding from external electromagnetic radiation as well as to provide an environment in which standardized electromagnetic radiation test procedures could be developed for many types of radiation sources. In order to determine the susceptibility of a device to radiation using these test chambers the device must be subjected to the radiation frequencies and field strengths which are representative of the environment in which it will operate. It is desirable that the fields used to test a device in this manner be statistically representative of the electromagnetic environment so that the level of radiation to which the device is subject can be accurately determined.
Several types of electromagnetic field chambers which may be used for these purposes are known. For example, test chambers having parallel plates and biconical antennas were commonly used in the high frequency band. Although the parallel plates used in these test chambers allowed for adequate field strengths, the wave impedance of the test fields provided was three hundred seventy-seven ohms. This was not representative of the wave impedances experienced in many actual device operating environments.
Anechoic test chambers were also known. The chambers are provided with a large number of cones on the walls, floor, and ceiling to minimize reflected energy. These anechoic test chambers tended to be very expensive and did not adequately represent the wave impedance found in many real life situations. Furthermore, at very high field strengths anechoic chambers had problems with power absorption and heat dissipation.
Mode stirred test chambers are another type of test chamber known in the art of electromagnetic field testing. A mode stirred chamber was a modified shielded room that ranged in size from small metal enclosures having a size of approximately two cubic meters to large shielded rooms. The distinguishing feature of the prior art mode stirred chambers was a paddle wheel tuner. These tuners were relatively large field-perturbing devices which were shaped like a paddle wheel and rotated within the test chamber. This irregularly shaped 10 metal structure caused large changes in the standing wave patterns within the mode stirred test chamber as it was rotated. In this way, the many simultaneously existing modes were "stirred" as described in "Evaluation and Use of a Reverberation Chamber for Performing Electromagnetic Susceptibility Vulnerability Measurements" by M. L. Crawford and G. H. Koepke in the National Bureau of Standards Technical Note 1092, April 1986.
Shielding effectiveness methods using the mode-stirred chamber were easy to perform and repeatable. The traditional approach, referred to as the discrete frequency method, involved testing one frequency at a time. In this method energy was concentrated at discrete frequencies thereby yielding extremely accurate data. However, testing time using this method was as long as forty minutes per frequency.
As is also true with other prior art test chambers, mode stirred test chambers were required to have dimensions equal to the wavelengths of the electromagnetic energy applied during the test in order for them to resonate at the test frequency. Therefore, tests performed using these mode stirred test chambers were limited to rather high frequency, short wavelength energy. Otherwise the size of the test chambers would be prohibitively large. A need therefore existed for test chambers which could be used to test 10 electrical equipment at low frequencies wherein the wavelength may be several hundred yards, thereby making it unfeasible to build traditional mode stirred test chambers large enough.
The mode stirred chamber of the present invention can be used to test electronic equipment in the range from 200 MHz to 40 GHz in the manner commonly used in the prior art stirred mode test chambers. However the stirred chamber of the present invention is also effective to test electronic equipment using high frequency near fields. Thus it is effective to generate near field impedance high frequency fields as well as far field impedance in the microwave region. This feature is provided by disposing at least one conductive metal rod, and preferably a plurality of such rods, between opposing walls of the test chamber. The conductive rod receives radio frequency energy applied to the test chamber and reradiates the received energy due to currents induced in the rods by the applied energy. When the rods are electrically coupled to the opposing walls they effectively form an equivalent loop antenna with the adjacent walls and reradiate magnetic fields within the test chamber in a manner known with respect to loop antennas. When the rods are insulated from one wall they reradiate electric fields in a manner known with respect to conventional whip antennas. When the electronic equipment under test is placed near the equivalent loop antenna the low magnitude wave impedance associated with its nearness to a magnetic field antenna is applied to the electronic equipment under test. Furthermore, when the electronic equipment under test is placed near the equivalent whip antenna the high magnitude impedance electric field associated with an antenna is applied to the equipment under test.